Among the functions of a suspension system is that of isolating the passengers of a vehicle from the jarring sensation caused by uneven road surfaces and also to keep the tire in contact with the road surface in order to maintain directional control of the vehicle. For many years, suspension systems have relied on passive systems of springs to absorb energy from encountering a bump and dampers to dissipate that energy.
Passive suspension systems are a compromise between occupant comfort and road holding ability. Active suspension systems can eliminate this trade-off since they can respond to vertical forces from the road independently of inertial forces on the body. Known active suspension systems use electromagnetic actuators at each wheel to apply vertical forces between the sprung car body and each unsprung wheel assembly of the vehicle.
A challenge that arises in active suspension systems is posed by the first vertical resonance of the unsprung wheel assembly. Without some way to suppress this resonance, the vertical contact force between tire and road can vary significantly. This, in turn, interferes with road holding ability of the tire, thus reducing the driver's directional control of the vehicle. In some cases, the un-damped resonant motion can grow large enough such that it reduces the contact force to zero. At that point, with the tire being momentarily airborne, the road holding capability of the tire becomes non-existent.
One such method to control this resonance without putting any of the damping force into the sprung body mass is to install a tuned vibration absorber, also known as a “mass damper” on each wheel of the vehicle. Packaging such a device within a wheel assembly is challenging. Known solutions have required relocating the brake caliper to a non-ideal location.